An all-reflective (or catoptric) optical system avoids chromatic aberration because there is no dispersion from a reflective surface as there is through a lens (which also produces transmission losses). A catoptric system thus allows a broad spectrum of electromagnetic radiation, from infrared to ultraviolet, to be focused identically within the limits of geometric optics.
Spherical aberration is produced by optical systems having different focal distances for different input ray heights, causing a rotationally symmetric blurred image of a point object. Comatic aberration, or coma, is produced when rays have different image heights depending on the distance of the ray from the center of the entrance pupil. The resulting image of a point object resembles a cone. Astigmatism is produced by optical systems having different focal distances in orthogonal directions, resulting in an image of a point ranging from a line to a circular blur, depending on the observation point.
An afocal optical system produces collimated exit beams from collimated entrance beams. A focal system produces focused exit beams from collimated entrance beams. The image of the aperture stop in object space is called the entrance pupil and is typically located at the primary mirror. The image of the aperture stop in image space is called the exit pupil.
Conventional afocal two-mirror and three-mirror systems, while providing aberration minimization, have the disadvantage of having a limited field of view when providing desirable diffraction-limited wavefront quality. Further, these conventional systems are typically overly long (having a length, or envelope, greater than three times the maximum pupil diameter) for many applications.
An example of a prior art two-mirror system is the Mersenne telescope which utilizes confocal paraboloidal mirrors producing an internal image. The field of view is quite narrow, typically 0.2.times.0.3.degree. for diffraction-limited wavefront quality at 1000 nm wavelength. Further, Mersenne telescopes have a low exit pupil clearance (less than one exit pupil diameter from the nearest beam) making coupling to subsequent image-forming systems difficult. An example of a three-mirror system is U.S. Pat. No. 3,674,334 to Offner. This is a concave-convex-concave mirror system with the convex mirror providing field curvature correction allowing a greater field of view in one dimension (for example, greater than 4.degree. total field) and greater exit pupil clearance. However, this three-mirror system has a limited field of view in the other dimension (for example, 0.1.degree..times.4.00.degree. for diffraction-limited wavefront quality at 1000 nm wavelength). This results in an extreme rectangular field of view. Further, the envelope length is approximately 2.5 to 4 times the maximum pupil diameter, making the system too long for many space-limited applications.
An example of a four-mirror paraboloidal system for avoiding aberration is disclosed in U.S. Pat. No. 3,062,101 to McCarthy. Incident light is reflected by a concave mirror to a fold-flat mirror which redirects the beam 90.degree. to a second concave mirror which is concentric with a third convex mirror and a fourth concave mirror, producing an image beyond the fourth mirror. Because of the concentric configuration, the fold-flat mirror causes obscuration which then necessarily limits the size of the fold-flat mirror and causes throughput limitations and diffraction effects on the image.
In summary, for many multiple mirror systems, there is the problem of possible obscuration caused by mirrors blocking rays and thereby decreasing both the amount of radiation gathered and the field of view while introducing adverse diffraction effects in the image. Further, multiple mirror systems, because of geometric requirements for achieving true images, may be overly large or bulky for many applications (such as airborne and outer space applications with their attendant packaging limitations). Still further, the entrance and exit pupils are typically not easily accessible, thereby causing other problems such as vignetting.